allow dwqe.c to build on architectures that do not have machine/fdt.h

allow dwqe.c to build on architectures that do not have machine/fdt.h

Move struct if_device to a new fdt-specific softc struct, along with
the gmac_id field which is only used by if_dwqe_fdt.c at present.
This avoids the need to include any fdt header files in dwqe.c.

ok kettenis@